The invention relates to an apparatus for detecting intruders. The intruder detector consists of a housing provided with at least one window, a passive infrared detector (for detecting the radiation emitted by an intruder around a wavelength .lambda..sub.1), and anti-obscuring means. The anti-obscuring means detects infrared radiation having a wavelength .lambda..sub.2 which indicates the presence of an obscuring of the apparatus for detecting intruders. The detector further includes as electronic means for operating an alarm system when the presence of an intruder or of an obscuring element has been detected.
A device of this kind is described in British Pat. GB 1,603,306 (corresponding to U.S. Pat. No. 4,242,669). This patent discloses a passive infrared apparatus for detecting intruders. It comprises a pyroelectric detector which detects the infrared emission produced by a living creature (and more particularly by an intruder entering, without authorization, a room to be supervised). The principle of such an apparatus is to detect variations of infrared emission. Variations are obtained by segmenting the scrutinization of the zone to be supervised by the use of a network of mirrors focusing the emitted infrared radiation on the pyroelectric detector. This emission has a maximum for wavelenghts of 8 to 10 .mu.m.
However, the disadvantage of a passive detection apparatus is that it is possible to partially or entirely obscure such an apparatus. In order to obviate this disadvantage, the British Pat. GB 1,603,306 utilizes a system detecting an obscuring element by detecting a second infrared radiation having a wavelength of 0.9 .mu.m emitted by an emitter.
The emitter and receiver at 0.9 .mu.m are arranged in the same housing as the pyroelectric detector and utilize for their operation the same entrance window. The principle of this anti-obscuring device is to determine the reflection coefficient of the obscuring element. The latter may be a leaf of paper or metal, a rigid obstacle, a projection of a pulverulent product or the like. In all these cases, the light emitted at 0.9 .mu.m by the emitter is reflected by the obscuring element and is retransmitted to the detector at 0.9 .mu.m located in the proximity. When such an obscuring operation is detected, electronic means cause an alarm to become operative.
There are many ways in which an obscuring operation can be effected and a large number thereof are not detected at all by the apparatus described in British Pat. No. GB 1,603,306.
In fact, the obscuring element may not have a sufficient reflection coefficient (i.e. may absorb the radiation at 0.9 .mu.m). For example, the obscuring element may be painted black. In this case, the detector at 0.9 .mu.m will not receive or substantially not receive light and will not detect the presence of the obscuring element.
Likewise, because the emitter and the receiver are stationary with respect to each other, even if the obscuring element has a sufficient reflection coefficient, the light may be reflected away from the direction of the detector. If the obscuring element is too close to the apparatus, the chances of detecting the obscuring are not equal to zero. However, if the obscuring element is arranged at a non-neglible distance in the form of an obstacle, it is not very probable that the reflected light reaches the detector at 0.9 .mu.m.
Now it is very easy to imagine situations in which obstacles can be arranged during a period in which the apparatus is inoperative. This will be the case in public or semi-public places in which an intruder can enter by day to obscure the detector when the system is stopped, and can return when the system will have been put in operation again for supervising then deserted places.
On the other hand, according to British Pat. No. GB 1,603,306, the apparatus will detect an absence of obscuring when no radiation at 0.9 .mu.m will have been detected by the detector at 0.9 .mu.m. It will thus be clear that, if either the emitter or the detector becomes defective, no signal will appear, which will be interpreted as a situation of non-obscuring.
The apparatus according to this Patent is consequently either not very reliable or inoperative in a large number of conventional situations.